1. Field of the Invention
The present invention relates to a wavelength division multiplexing (WDM) optical transmission system and transmission method for use in the field of optical communications.
2. Description of the Related Art
Conventionally, a WDM optical transmission system that transmits signals using a plurality of wavelengths is used in order to increase transmission capacity in optical communication.
FIG. 16 is a structural view showing a conventional WDM optical transmission system. In the transmitting section, a wavelength multiplexer 8-1 couples output signals from a plurality of optical transmitters 1-1 having different wavelengths (=optical frequencies). The coupled plurality of optical signals are transmitted using a single common optical transmission medium 1-4. In the receiving section, a wavelength demultiplexer 8-2 demultiplexes received optical signals into optical signals having the respective wavelengths, and a plurality of optical receivers 1-6 receive the optical signals of respective wavelengths. A WDM optical transmission system is achieved using the above structure.
From the standpoint of effective utilization of the optical wavelength bands, dense WDM optical transmission in which the spacings between optical wavelengths are close together is desired. If the optical frequency spacing is taken as Δf [Hz] and the transmission speed is taken as B [bit/s], then B/Δf [bit/s/Hz] gives what is known as the spectral efficiency. The theoretical limit of the spectral efficiency of a scheme in which ON/OFF modulation (intensity modulation) of double sidebands is performed (namely, a scheme in which double sidebands are generated by ON/OFF modulation) is 1 [bit/s/Hz].
In a conventional WDM optical transmission system, the receiving section extracts the desired optical signal by means of a wavelength selection filter that uses a wavelength demultiplexer such as an arrayed waveguide grating. However, if an attempt is made to transmit a plurality of signals at high density, because the signals of adjacent wavelengths overlap, the problem arises that the wavelength selection filter cannot separate the desired signals. If the wavelength spacings are broadened in order to prevent interference between adjacent wavelengths (adjacent frequencies), the spectral efficiency is reduced. In a conventional WDM optical transmission system, the spectral efficiency is normally approximately 0.4 [bit/s/Hz] or less.